1. Field of the Invention
The present invention relates to a water-based ink composition, an ink set and an image forming method.
2. Description of the Related Art
In recent years, by enhancement of needs such as source protection, environment conservation, and improvement in stability of work, conversion of a paint and an ink into an aqueous paint and an aqueous ink is progressing. Quality required for the aqueous paint and the aqueous ink includes fluidity, storage stability, luster of a film, clearness, and a coloring force like an oily paint and an oily ink. However, since a majority of pigments are remarkably inferior in suitability such as pigment dispersibility in an aqueous vehicle as compared with oily ones, satisfactory quality is not obtained by a normal dispersing method. Use of various additives, for example, a resin for an aqueous pigment-dispersion and an surfactant has been studied, but an aqueous paint or an aqueous ink which satisfies the all suitabilities, and is comparable to the current oily paint or oily ink having high quality has not been obtained.
In order to solve such problems, for example, a water-based ink composition containing a coloring material covered with a water-insoluble polymer as a colorant, and a polymer particle is disclosed (see e.g. Japanese Patent Application Laid-Open (JP-A) No. 2001-329199). An inkjet ink using the water-based ink composition is said to be excellent in water resistance, scratch resistance, marker resistance, and printability.
In addition, an ink composition containing at least a coloring material covered with a water-insoluble polymer as a colorant, and a resin emulsion as an additive, wherein a weight average molecular weight of the water-insoluble polymer is 50000 to 150000, and the resin emulsion includes a polymer having constituent components having the same structure as that of a water-insoluble polymer, and having a molecular weight expressed as the weight average molecular weight which is 1.5 times to 4 times the weight average molecular weight of the water-insoluble polymer is disclosed (see e.g. Japanese Patent Application Laid-Open (JP-A) No. 2006-273892), and the ink is stated to be good in luster and scratch resistance.